These are great posts. The Wiki article that Scott posted is good and Colorado David's description is perfect. The point of all this is to compose an image such that the viewer's eyes are drawn into the photo and allowed to "walk around" in it. You don't want to anchor their eyes in the center. Colorado David also speaks about tension in the photo, as he pointed out that you wouldn't place someone looking to the right in the lower right corner. It would feel more natural to have them looking to the left from the right hand side or vice versa, so that the subject is looking into the frame. Now this is just a guide, and I've seen several fine examples where people break the rule.